For the transmission of electrical power, cables are often identified by the voltage levels they are designed to carry. Medium voltage cables are generally those intended for carrying 1 kV-35 kV, and high voltage cables are intended to carry above 35 kV.
Medium and high voltage cables can be installed in a number of settings. For example, they may be buried underground, installed in small passageways such as in manholes or ducts, or mounted in cable racks. When installed in manholes or ducts, the cables may be secured to a support structure. The securing helps confine the cables to a desired space, prevents movement of the cable in the manhole or duct when other portions of the cable are manipulated, and helps resist buckling of the cable that can occur when electrical power is applied to the cable.
Clamps are conventionally used to secure the cable to a support structure within manholes or ducts. To be effective, clamps must be robust, easy for the installer to use, and should accommodate a variety of cable sizes. Most clamps for medium and high voltage cables comprise two pieces. Generally, these two pieces are half-shells that, when mated during cable installation, surround the circumference of the cable at a particular longitudinal position of the cable. The bottom half-shell can be secured in some fashion to a support structure within the manhole or duct, preferably before positioning the cable. The cable is then placed so the bottom half-shell surrounds a portion of the cable, after which the top half-shell of the clamp is placed on the cable to surround its remaining portion. The two half-shells are then tightened together, thereby securing the cable. This tightening can be performed, for example, by some type of bolt arrangement serving to couple the bottom half-shell to the support structure.
FIG. 1 depicts a partial cross-section of a conventional two-piece cable clamp 1. The clamp has two primary pieces 3 and 5 that form half-shells to surround a cable when installed. The top half-shell 3 has flanges 7 through which a bore 11a is present, and the bottom half-shell 5 has flanges 9 through which a threaded bore 11b is present (as shown in FIG. 1 for one side of clamp 1). The threaded portion 13 of the bolt 17 engages into the lower end of the threaded bore 11b to secure the bottom half-shell 5 against a support structure (not shown). After bottom half-shell 5 is bolted, the cable may be positioned and half-shell 3 arranged over the cable. Another bolt 18 passing through the bore 11a secures top half-shell 3 to bottom half-shell 5 by engaging its threaded portion 15 into the upper end of the threaded bore 11b. Each half-shell 3 and 5 is fitted with cushioning 19 so as not to damage the cable when the clamp is tightened and to accommodate thermal expansion of the cable during cycling.
Applicant has observed that the clamp of FIG. 1 has a number of disadvantages. First, it is expensive, especially due to the threaded portions and the number of distinct parts comprising the clamp. The rubber cushioning 19 in clamp 1 also can degrade over time, shortening the life of the clamp. In addition, a clamp of the type depicted in FIG. 1 has a small number of cable sizes that it can accommodate. To secure different cable diameters, suitably sized bottom and top half-shells must be selected. For example, the clamp of the design in FIG. 1 requires at least fifteen different sizes for its half-shells to accommodate a cable diameters ranging from 70 mm to 146 mm.
Korean Patent Application Publication No. KR 2004/0015843 illustrates another design for a cable clamp having two half-shells. The bottom half-shell of that clamp is provided with two pairs of flanges. The first pair of flanges at the base of the half-shell are provided with bores. A pair of bolts extends through said bores and through a support structure. The bolts, and bottom half-shell, are then secured to the support structure with respective nuts.
The second pair of flanges is near the upper portion of the bottom half-shell. These are also provided with bores which mate to corresponding bores provided in a pair of flanges of the top half-shell. Each flange of the top half-shell also includes a chamber housing a spring assembly. The bolts extend through the bores provided in the top half-shell flanges and through the upper flanges of the bottom half-shell. Nuts can then be placed on the bolts to secure the top half-shell to the bottom half-shell. As a result, a cable can be secured between the bottom and top half-shells under a spring-loaded compression. While the spring assembly allows a range of cable sizes to be used with one cable clamp and eliminates the threaded inserts, the clamp design is relatively expensive due to the number of parts and manufacturing difficulties associated with the somewhat complicated casting of the chamber on the flanges of the top half-shell.
FIG. 2 depicts a side view of another conventional two-piece cable clamp 20. Clamp 20 employs a pair of springs 22. Both half-shells 24 and 26 are identical in size and shape. Both half-shells are provided with a pair of bore holes (not shown). A single bolt 28 passes through each bore hole on respective sides of top half-shell 24, through a corresponding bore hole of bottom half-shell 26, and through a support structure 30. A nut can then secure the bolt 28 and the clamp 20 to support structure 30.
Clamp 20 accommodates a limited range of cable diameters. For example, five different clamp sizes are needed to accommodate cable diameters from 70 mm-146 mm. Additionally, Applicant has found that, at least with respect to larger cable sizes, the half-shells are not sufficiently robust and may bend under prolonged clamping. Further, because clamp 20 relies on the same bolts and nuts to secure bottom half-shell 26 to support structure 30 and to tighten top half-shell 24 to bottom half-shell 26, installation is difficult. In particular, bottom half-shell 26 cannot be locked into place against support structure 30 before a cable 32 is secured into clamp 20 by the top half-shell 24.
Applicant has observed that known designs for cable clamps are not satisfactory for securing medium or high voltage cables within confined spaces such as manholes or ducts. Those designs lack the ability to mount cables with simple and convenient steps, require multiple parts to accommodate different cable diameters, and do not minimize the number of parts within the clamp assembly. They also lack the robustness required for fitting a single clamp assembly to a wide range of different sized cables without substantial risk of failure.